Police and council officers are now inspecting the pool's fencing to see if it was up to standard at the time.

Police are also considering whether the girls used an outdoor chair or even a trampoline to break into the pool area.

The deaths are not being treated as suspicious.

Peter Brown, the grandfather of the two girls, said the parents are devastated.

"He's cut up something terrible, and she's just as bad, because she had to dive in the pool to get them out," he said.

"They'd just finished cleaning the pool to go swimming, it had regulation fencing all around it, so I don't know how they got in."

Heartbroken aunty Tamara Sengstock told Fairfax the mother-of-five was distraught but trying to hold it together so she could care for her son, and two other daughters -- an 18-month-old and a newborn.

"I can't believe you beautiful angels were taken so soon and only were 4 and 3yrs old," she wrote on Facebook.

Young sisters drowned in family pool tragedy

TWO young sisters have reportedly died in a pool accident today.

The two drowned in their family pool in Kingston, according to tweets by Nine News and the ABC.